The invention relates generally to sheet feeding apparatus and more particularly to improvements in the sheet lifting, sheet separation and feeding, and control systems for implementing the lifting, separation and feeding
Sheet feeding devices are old in the art and many of the earliest are concerned with the feeding of single sheets of paper from a pile or stack and were conceived as adjuncts for printing presses or other printing related operations. Many of the these prior art devices employed one or more vacuum pickups for lifting and separating the top most sheet from the pile. Amongst these are the U.S. patent of Payne, et al, U.S. Pat. No. 1,391,271 which employs a vacuum bar to lift up the rear of the top most sheet and an endless conveyor to complete the lifting of the sheet and move the lifted sheet to either feed rolls or a machine table.
While many of the prior art devices performed well enough for their intended use; they have failed to perform adequately when the individual sheets of the pile adhere to one another or the sheets are relatively stiff such as metal sheets The condition where the sheets adhere to each other is frequently encountered, especially with metallic or plastic sheets and laminated sheets. The adhesion may be due to any of a number of factors including static electricity, cohesion, vacuum, liquid film adhesion, adhesives and surface tension. Further, the relative inflexibility of metallic, plastic or laminated sheets renders most, if not all of the systems intended for paper and similar materials, inoperable.
There is an especially great need for sheet or strip feeding in contemporary automated machine systems which automatically and accurately position sheets or strips in a machining area for repetitive punching, stamping, component mounting, etc., operations. One such system is described in the co-pending U.S Patent Application of Samuel P. Willits, et al, Ser. No 6/920587, assigned to the same assignee as this Application. Such systems literally "eat-up" strips of material and their use would be considerably less advantageous if they could not be regularly and rapidly resupplied with sheets or strips.
As set forth above, prior art sheet feeding devices have proven either unreliable or inoperative when faced with stiff plastic, metallic or laminated sheets and particularly so when the sheets adhere to each other. While prior art devices have attacked these problems, none have overcome these problems in a single device and provided an adaptable sheet feeding system or device for feeding automated contemporary manufacturing process equipment.